1. Field of the Invention
The present invention relates to a window for vehicles, particularly automobile vehicles, having improved acoustic insulation properties, especially with respect to noise of structure-borne origin.
2. Discussion of the Background
Soundproofing windows are used not only for windows in buildings but also to an increasing extent in automobiles. Although soundproofing windows for buildings can be relatively thick, laminated windows used for automobile construction usually have thicknesses not exceeding about 6 mm. Consequently it is advisable to use, as an intermediate layer between the two sheets of window glass, a viscoelastic polymer that imparts a highly effective anti-noise effect even in relatively thin layers. In addition, the polymer must also satisfy, over the long term (meaning for the entire life of the vehicle) all conditions imposed on polymers used in automobile windows. These conditions include in particular low cloudiness, high transparency and good ultraviolet resistance. In addition, these polymers must ensure high-quality and durable assembly with the adjacent layers and must retain their good noise-damping properties even at high and low temperatures. Finally, the anti-noise layers must not impair the safety-glass properties of the window. Acrylic viscoelastic polymers have proved particularly appropriate as antinoise layers.
EP 0 532 478 A2 teaches a soundproofing laminated window that is also suitable as an automobile window and that comprises an intermediate layer of viscoelastic acrylic polymer. In the case of this known laminated window, the intermediate layer separating the two glass sheets is formed by a polymerizable monomeric composition comprising 5 to 50 weight % of an aliphatic polyurethane and 15 to 85 weight % of a photopolymerizable mixture of different acrylic monomers and common polymerization additives. The mixture of monomers is admitted into the space separating the two glass sheets, and polymerization by ultraviolet radiation is initiated. This known soundproofing laminated window is not suitable for series production, however, because its process of production (polymerization of the mixture of monomers introduced between the glass sheets) is relatively expensive.
In the case of the industrial process for production of laminated windows, assembly of the two glass sheets with a prefabricated polymer film is generally performed at elevated temperature and under pressure. A process of this type intended for production of laminated windows having good noise-damping properties is known from EP 0 457 190 A1. In the case of this known process, a prefabricated polymer film having a high noise-damping coefficient and comprising at least two layers, one of which is made from a first specified polyvinyl acetal and plasticizer and the other of a second specified polyvinyl acetal and plasticizer.
Viscoelastic acrylic polymers with good noise-damping properties are also known in the form of thin films. These anti-noise films can be used for production of laminated windows by interposing them between two thermoplastic films comprising in particular polyvinylbutyral and bonding them to these films as well as to two external glass sheets by the standard process for production of laminated windows by assembly at elevated temperature and under pressure. Soundproofing windows of this type have a good noise-damping coefficient, but it has been found that in the course of time the acrylic polymer becomes cloudy and the noise-damping properties deteriorate, with the result that the laminated window thereby becomes useless.
On the other hand, among all the qualities contributing to comfort in modem transportation means such as trains and automobiles, silence is becoming the determining factor. In fact, the other sources of discomfort stemming from mechanical, thermal, visibility, and other considerations have been gradually mastered. Improvement of acoustic comfort presents new difficulties, however; noises of aerodynamic origin, in other words noises created by friction between the air and the moving vehicle, have been treated successfully, at least in part, at their source, or in other words, to economize on energy, the shapes have been modified, penetration into the air has been improved, and turbulence effects, which themselves are noise sources, have been reduced. Among the vehicle walls that separate the source of external aerodynamic noise from the inside compartment in which the passenger is situated, windows are obviously the most difficult to treat. Pasty or fibrous absorbing materials intended for opaque walls cannot be used and, for practical or weight reasons, the thicknesses cannot be increased. European Patent EP B1 0 387 148 teaches windows that achieve good insulation from noise of aerodynamic origin without excessive increase in their weight and/or thicknesses. The patent thus teaches a laminated window in which the intermediate layer has a flexural damping coefficient of ν=Δf/fc larger than 0.15, the measurement being performed by shock excitation of a laminated bar of 9 cm length and 3 cm width made of a laminated glass in which the resin is located between two glass members each 4 mm thick, and by measuring fc, the resonance frequency of the first mode, and Δf, the peak width at amplitude A/√{square root over (2)}, where A is the maximum amplitude at frequency fc, such that its acoustic attenuation index does not differ for any of the frequencies higher than 800 Hz by more than 5 dB from a reference index, which increases by 9 dB per octave up to 2000 Hz and by 3 dB per octave at higher frequencies. In addition, the standard deviation a of the differences of its acoustic attenuation index relative to the reference index is always smaller than 4 dB. The thicknesses of the two glass members can be identical and equal to 2.2 mm. This patent therefore proposes a general solution to the problem of acoustic insulation of the aerodynamic noises of a vehicle.
However, the noises themselves such as engine noises, bearing noises or suspension noises must be treated at the same time. These noises have already been treated at their origin or to some extent during their propagation, whether airborne (especially absorbing lining) or structure-borne (joints of elastomer, for example). For windows, European Patent EP B1 0 100 701 teaches windows that achieve good soundproofing of highway noises, or in other words good insulation of noises during their airborne propagation.
One of the windows according to this patent comprises at least one laminated window, and the resin of the laminated window is such that a bar of 9 cm length and 3 cm width made of a laminated glass comprising two glass sheets of 4 mm thickness joined together by a 2 mm layer of this resin has a critical frequency that differs by at most 35% from that of a glass bar having the same length, the same width and a thickness of 4 mm. The windows according to this patent have an excellent acoustic attenuation index for highway traffic.
In contrast, the treatment of windows to combat noises of structure-borne origin, meaning noises transmitted through solids, is more difficult to achieve. In fact, it turns out that the use of joints is still insufficient to prevent transmission of noise by window vibration. In this connection, it has actually been observed that, at certain engine speeds, perceptible buzzing is felt by the passenger, thus causing a source of discomfort. In fact, the engine rotation leads to the development of vibrations which are transmitted, for example, to the car body and then by chain effect to the windows. It is known that the energy acquired by an object subjected to a shock causes a vibration phenomenon, and that, immediately after the shock, once the object has become free again, it vibrates in its natural mode. A vibrational frequency is associated with each mode. The vibrational amplitude depends on the initial excitation, or in other words on the spectral component of the shock (amplitude of the shock at the frequency under study) and of the impact zone of the shock, the magnitude of the modal deformation depending on whether the shock occurred at a neutral point or vibration node.
The following conditions must be met for a natural mode to be excited:    (1) the deformation caused at the point of impact is not situated at a vibration node of the mode,    (2) the energy spectrum of the shock has a component at the resonance frequency of the mode.
This second condition is almost always met, because a very brief shock has a practically uniform energy spectrum.
The first condition is also met and, for a bar free at its ends, for example, it is sufficient to tap one of the ends to excite all modes.
In the application in question, structure-borne excitation is peripheral and the inventors have demonstrated that, at certain engine vibration frequencies, in other words at certain engine speeds, the windows and the passenger compartment of the vehicle each have a vibration mode, the coupling of which amplifies the window buzzing caused by radiation of noises originating in this case from the engine. Of course, the engine speed causing these phenomena is particular to each type of vehicle, and therefore cannot be generalized to a single value.